(a) Field of the Invention
The present invention relates to a laser diode for generating frequency-tunable optical pulses, used for 3R regeneration (re-timing, re-shaping, and re-amplifying) of restoring an optical signal deformed while being transmitted through an optical fiber to its original state, and for generation of high bit-rate optical signals. More particularly, the invention relates to a self-mode locked multi-section semiconductor laser diode that can be fabricated on a single substrate and that includes a complex-coupled DFB (distributed feedback) laser section.
(b) Background of the Related Art
In an optical communication system, an optical signal transmitted through an optical fiber is subjected to reduction of its magnitude and temporal deformation due to dispersion while it is being transmitted. To correct this to restore the optical signal to its original state, 3R regeneration (re-timing, re-shaping, and re-amplifying) is required. Among this 3R regeneration, re-timing is extracting a clock from the deformed signal to obtain a restored signal from the clock and a deformed optical signal through a decision element. Methods of extracting the clock include electrical and optical methods.
A. A. Tager published the theoretical background of generation of optical pulses according to a short external-cavity laser diode, entitled “High-frequency oscillations and self-mode locking in short external-cavity laser diodes” in IEEE J. Quantum Electron, Vol. 30. According to this article, high-frequency optical pulses can be generated according to self-mode locking of compound cavity modes. Specifically, this article discloses that an optical pulse of tens of GHz can be acquired by self-mode locking of compound cavity modes by appropriately controlling the light beam strength and phase variation when a laser beam output from the single-mode laser diode is propagated through an external cavity and again fed back to the laser diode in the single-mode laser diode structure including the short external cavity having a length less than several millimeters.
S. Bauer published the self-mode locked DFB laser diode structure in Electron. Lett. Vol. 38, issued in March of 2002. This laser diode has an index-coupled DFB laser section and an external cavity including a phase control section and an amplifier section. The strength and phase of a light beam, which propagates through the external cavity and is then fed back to the DFB laser region, are controlled by injection currents of the amplifier section and the phase control section, and the injection currents are varied to obtain a wide frequency tuning range of tens of GHz. However, this technique may adversely affect the generation of an optical pulse and stable frequency variation because there is a probability of occurrence of mode hopping or multi-modes in the index-coupled DFB laser section according to a feedback phase variation in actual applications.